add experimental warning to kraft docs

Author: maltesander

docs/modules/kafka/pages/usage-guide/kraft-controller.adoc

@@ -1,6 +1,8 @@
-= KRaft mode
+= KRaft mode (experimental)
 :description: Apache Kafka KRaft mode with the Stackable Operator for Apache Kafka
 
+WARNING: The Kafka KRaft mode is currently experimental, and subject to change.
+
 Apache Kafka's KRaft mode replaces Apache ZooKeeper with Kafka’s own built-in consensus mechanism based on the Raft protocol.
 This simplifies Kafka’s architecture, reducing operational complexity by consolidating cluster metadata management into Kafka itself.